1. Field of the Invention
A lithium-ion secondary battery is small in size and light in weight compared to a lead storage battery or a nickel-hydrogen battery in the related art. The lithium-ion secondary battery has characteristics in which a voltage of a single cell is high, energy density is high, a memory effect is small, and self-discharge is small. Due to these characteristics, the lithium-ion secondary battery is mounted on an apparatus such as a cellular phone and a notebook personal computer that are small in size and are portable. Furthermore, the lithium-ion secondary battery is starting to be used for a part of hybrid cars.
2. Description of the Related Art
It is necessary to increase a capacity of each of a positive electrode and a negative electrode so as to increase energy density of a battery. However, in a part of negative electrodes, an effective capacity of graphite, which is used in current batteries, is approximately 360 mAhg−1, and this value is approximately compatible to a theoretical capacity (372 mAhg−1). Therefore, it is necessary to change a material so as to increase the capacity of the negative electrode.
Examples of a material of a negative electrode in which a capacity is high and a discharge electric potential is low include silicon (hereinafter, referred to as “Si”). However, in Si a volume variation is large during charging/discharging, and miniaturization of particles progresses during repetition of a charging/discharging cycle, and thus contact properties between a current collector and a conductive auxiliary agent decreases. Therefore, there is a tendency for deterioration of characteristics to easily occur.
Therefore, various methods are suggested to suppress deterioration of Si during charging/discharging cycles. For example, as described in Japanese Unexamined Patent Application Publication No. 2010-108915, a method of restricting a used ratio of Si may be exemplified. However, when the used ratio of Si is restricted, a substantial discharge capacity is apt to decrease. Therefore, the deterioration of the characteristics in the case of repeating the charging/discharging cycle may not be prevented. In addition, no review for improving stability of a battery using Si has been made until now.